The advent of the protease inhibitor era in AIDS treatment has brought with it substantial reductions in plasma HIV-1 RNA concentration, increases in CD4 cell count and clinical improvements over previously available therapies and has radically altered the goals of treatment. Despite these advances, approximately 10-50% of subjects may not achieve a complete virologic response or may fail virologically after initially achieving a plasma HIV-1 RNA level below the limit of quantification of currently available assays. Given the recognized difficulty in achieving virologic suppression in individuals who have experienced virologic failure, this study has been designed with the purposes of: a) providing maximally potent, albeit complex regimens utilizing combinations of currently approved protease inhibitors and experimental agents drawn from the protease inhibitor (amprenavir), nucleoside (abacavir), nucleotide (adefovir dipivoxil), and non-nucleoside (efavirenz) reverse transcriptase inhibitor classes of agents; b) investigating the salvage potential of amprenavir in dual protease inhibitor containing regimens; c) determining if the potential for salvage is greater if a moderately liberal definition of virologic failure (i.e., 1,000 copies/ml plasma HIV RNA) is employed; and d) determining if a strict definition of virologic success in the setting of protease inhibitor failure (i.e., achieving a plasma HIV-1 RNA level <200 copies/ml) is realistic with the therapeutic agents on the immediate horizon. This will be a Phase II, randomized, partially placebo-controlled, 4-arm trial comparing amprenavir (APV) in a single-Protease Inhibitor (PI) regimen versus APV in combination with saquinavir (SQVsgc), indinavir (IDV) or nelfinavir (NFV) in HIV-infected subjects currently failing IDV, RTV,SQV(sgc or hgc) or NFV, as reflected by a plasma HIV-1 RNA concentration of greater than or equal to 1,000 copies/ml. All subjects will receive amprenavir (APV), abacavir (ABC), efavirenz (EFV) and adefovir dipivoxil (ADV). Subjects will be selectively randomized to one of four treatment arms. Study Objectives: Primary: 1) to compare the proportion of subjects achieving a plasma HIV-1 RNA concentration below 200 copies/ml across the study arms at 24 weeks. 2) to compare the time to quantifiable viremia across the study arms. 3) to compare safety and tolerance across the study arms."